Several key features in the synoptic-scale environment (in addition to those mentioned in Section 2 above) contributed both to the initiation of the Furnas County storm and to its evolution into a cyclic supercell. The synoptic-scale environment also created a low level shear environment so favorable for supercell tornadogenesis that the absence of boundaries in the near-storm environment did not deter the formation of tornadoes. Such boundaries have been identified as key players in many significant supercell tornado outbreaks (see, e.g., Markowski et al. 1998 and Rasmussen et al. 2000).
During the afternoon hours of 22 May 2004, a strong shortwave trough (seen in the height fields in Figs. 9a and 9b) in the middle and upper troposphere dominated the region. This trough was associated with substantial upward motion in the middle troposphere over the southern and central Nebraska.

Pressure falls ahead of the trough (not shown) were associated with the development of a complex surface low pressure system (Fig 3). The circulation around this center caused the development a wave along the front and encouraged the progression of a dry line bulge into northwest Kansas and southwestern Nebraska.

A number of factors focused synoptic and sub-synoptic scale lift in the lower mid troposphere over southern Nebraska on March 22. First, differential cyclonic vorticity advection and warm advection were associated with quasigeostrophic forcing for mid-tropospheric layer lifting over the area. The quasigeostrophic effects can be diagnosed by the area of 700 mb cyclonic vorticity advection by the thermal wind inferred from Fig. 9c.

Second, solenoidal lifting along and just south of the synoptic-scale boundary (Fig. 3) and roughly coincident with the greatest temperature gradient at 700 mb (Fig. 9c) probably was an additional source of lift. Finally some weak mid-troposperhic upward vertical velocity, associated with convergence extending south into Kansas along the dry line bulge (Fig. 3), is also evident in Fig. 9b.

Above 850mb, west-southwest winds had brought a warm, dry layer over the underlying very moist air mass at the surface. The resulting cap was very strong south of the Kansas border, and was roughly demarked by the 11oC 700 mb isotherm (Figs. 3 and 9a).

The presence of strong winds in the mid and upper troposphere (not shown) also helped create a favorable deep layer (i.e., 0-6 km) shear environment for supercells over Nebraska. But, of special note, is the area of markedly backed strong southeasterly flow that extended from extreme southwest Nebraska eastward just north of the Kansas border (Fig. 3). This flow environment created low level shear values (e.g., 0-1 km) of high magnitudes, consistent with those experienced across outflow boundaries.

3.2 Thermodynamic and Shear Environment

The thermodynamic environment featured moderate to strong instability over central and southern Nebraska with surface based Convective Available Potential Energy (sbCAPE) of between 2000 and 3000 J/kg at 0000 UTC (Fig. 10). The greatest values were associated with the tongue of high dew points that had curled around the dry line bulge discussed above (Fig. 3). It is interesting to note that region experiencing values of Convective Inhibition Energy (CINH) suggestive of a strong capping inversion or lid (values of <-50 J/kg) were coincident with the area experiencing 700 mb temperatures of roughly 11oC or greater (see Fig. 3). Thus, the values shown in Fig. 10 suggest that little or no inhibition existed for surface based convection in the area of high sbCAPE values in southern Nebraska in the afternoon hours of 22 May.

The KTOP (Topeka, KS) rawinsonde ascent was closest to the Furnas County storm at 0000 UTC on 23 May (Fig. 11a). This ascent was deep in the warm air south of the boundary, and showed the lid clearly (Fig. 3), while the KOAX (Omaha, NE - Fig. 11b) sounding was just north of the synoptic scale boundary and away from the lid edge.
The greatest difference between the two soundings is the warmer temperatures in the 850-700 mb layer at KTOP. These were associated with the surge of the elevated mixed layer (EML) northeastward over Kansas during the day. The thermodynamic profile in the vicinity of the Furnas County storm would have been a merge of the two soundings, and without the lid.

Another important point that should be made is that only in the region between the synoptic scale boundary (evident on Fig. 3) and the Kansas border was the dew point depression small enough to yield Lifting Condensation Levels (LCL) very close to the ground (<4000 feet) and also very close the parcel’s Level of Free Convection (LFC). Recent studies have shown that tornadogenesis is favored in areas with small temperature and dew point spreads (Davies 2004).

The wind information plotted on Fig. 11 is consistent with the locations of KTOP and KOAX relative to the synoptic-scale boundary. KOAX reported northeast winds in the convective boundary layer, since that location was north of the synoptic-scale boundary. KTOP had south southeasterly winds in the same layer.

To construct a proximity hodograph for the Furnas County storm, the authors used the Vertical Azimuth Display (VAD) wind profile for KUEX at 2330 UTC. Unfortunately, there were many missing levels for earlier times, including the time of tornado T1. However, the hodograph would have captured the shear characteristics both of the environment at the time of tornadoes T2 and T3, and of the environment into which the Furnas County storm was moving as it became cyclic.

The hodograph (Fig. 12) is striking. It contains a large anticyclonic loop from 0-3 km. Such a large loop is associated with dynamic pressure forces creating strongly deviant right moving supercells, often associated with mesocyclone tornadoes (Rotunno and Kemp 1985).

The motion of hypothetical splitting storms (given the wind profile shown in the hodograph) is also plotted on Fig. 12. According to the algorithm used to estimate the predicted storm motion (Bunkers et al. 2000), the left moving storm’s motion vector should have been on the hodograph; the left movers would not be supercells and would be suppressed. These characteristics were verified by the radar animations examined by the authors (not shown) that showed that the left moving storms in southern Nebraska had brief life cycles and tended to be weak.

A remarkable feature of the hodograph is the intense low level shear suggested by the anticyclonic loop, with a strong kink between 1 and 2 km. Such a kink was observed in the VAD-derived hodographs in Oklahoma on the day of the May 3, 1999 tornadic supercell outbreak (Thompson and Edwards 2000).

The calculated 0-1 km shear from Fig. 12 is 8.0 x 10 –2 s-1! Such shear, if tilted into the vertical and stretched, would be converted into vertical vorticity with a magnitude comparable to that found with weak and moderate tornadoes (Rasmussen et al. 2000). Rasmussen et al. (2000) found such values along boundaries, and presented a case study example of a non-tornadic supercell becoming tornadic when it intercepted and “ingested” such boundaries.

In the present case, the low level shear values were so great that any supercell that formed in such an environment would rapidly process through the so-called supercell cascade to tornadogenesis. In addition, as long as such a storm continued to move through such a favorable buoyancy and shear environment it would continue to produce tornadoes cyclically.

The low level shear favorable for formation of mesocyclones and tornadogenesis is also dramatized by the 0-3 km Storm Relative Helicity (SREH) of 516 m2 s-2. This value exceeds the values nominally considered favorable for the development of strong mesocyclones (Davies-Jones et al. 1990) and

is obviously consistent with what was observed in the radar evolution of the storm. The area in Nebraska that had values of 0-3 km SREH > 200 m2 s-2 at 0000 UTC 23 May 2004 is highlighted on Fig. 3.

Finally, the deep layer shear (0-6 km) calculated from the hodograph shown in Fig. 11 was favorable for supercells (Weisman and Klemp 1986). The deep layer shear is a simultaneous control on both (a) the storm ventilation, ensuring that the precipitation core does not interfere with the updraft; and (b) provides adequate environmental deep layer shear favorable for the development of mid-level mesocyclones. In the present case, the value of deep layer shear was 10.5 x 10 –3 s-1, and considered strong (Weisman and Klemp 1986).

A summary of the parameters discussed in this section is included in Table 1. The parameters are based upon an evaluation of the KTOP and KOAX soundings (Fig. 11), NCEP reanalysis sbCAPE and CINH (Fig. 10) and the proximity hodograph of KUEX VAD wind profile information (Fig. 12)